There have been known image display devices (such as liquid crystal displays, projectors, or organic EL displays) which can display images using four colors, including red (R), green (G), and blue (B), and in addition, white (W). In the present specification, such display devices will be called RGBW-type display devices. The three colors of red, green and blue are obtained by having white light pass through color filters of red, green and blue, respectively. The light is attenuated when passing through the color filters, making it difficult to raise the luminance of the image display device. For white, no color filter is needed (it is only necessary to provide a transparent filter), and attenuation due to the color filter does not occur. As a result, RGBW-type display devices are advantageous in that they can realize a higher luminance, compared with image display devices using only the three colors of red, green and blue.
In addition, RGBW-type display devices, such as liquid crystal display devices can achieve the same level of luminance as image display devices using only three colors of red, green and blue, by use of a lower backlight illuminance. Accordingly, the power consumption can be reduced.
The input pixel data supplied to a display device usually consist of data of three colors of red, green and blue. Therefore, in an RGBW-type display device, the three-color data of red, green and blue need to be converted to four color data of red, green, blue and white, before display. If this conversion is not properly made, for instance, white may be mixed in all the display colors, resulting in whitish images, and the color display may differ from what is intended by the input pixel data.
Patent Reference 1 and Non-patent Reference 1 disclose schemes for converting three-color data of red, green and blue to four-color data of red, green, blue and white, by which the brightness is improved without changing the hue and saturation.